Blast disease is a serious disease in plants such as rice and is caused by the rice blast fungi, Magnaporthe grisea. The disease has substantially damaged the rice yields in Japan and many other rice-breeding countries. The damage is particularly severe at low temperatures and in high humidity. The disease has been obviated by breeding resistant varieties as well as applying agricultural chemicals. Originally, there were rice strains resistant to the disease. These strains and varieties carry genes resistant to a specific race of the blast fungi, and these genes have been analyzed for a long time. Presently, about 30 genes have been identified as being blast-disease resistant (Kinoshita, Rice Genet. Newsl. 7:16-57 (1990), Iwata, Rice Genet. Newsl. 13:12-35 (1996), Iwata, Rice Genet. Newsl. 14:7-22 (1997)). These genes have been utilized to breed highly resistant varieties, and in consequence, a number of resistant varieties have been bred. However, the introduced resistance genes are becoming ineffective due to the emergence of novel races of the blast fungi (collapse of resistant varieties). Furthermore, the molecular mechanisms of expression of the blast disease resistance and the interaction between the rice blast fungi and resistance genes remain unknown.
The resistance gene Pi-b is located at the end of the long arm of rice chromosome 2 and displays resistance to all races of blast fungi identified in Japan except for 033b (Table 1).
TABLE 1 Fungal strain Ine Cho 2101 Ine THB9 Cho F67- Ine P-2b Ai74 - Variety Gene #003 #007 #013 #031 #033 b+ #035 #047 #101 #303 #477 Shin 2 -- S S S S S S S S S S Aichiasahi Pi-a S S S R S R S R S S Inabawase Pi-i R S R R R S S R R S Kanto 51 Pi-k R R S S S S R R R S Tsuyuake Pi-km R R R S S S R R R S Fukunishiki Pi-z R R R R R R S R R S Yashiromochi Pi-ta R R R R R R R R S R Pi No.4 Pi-ta2 R R R R R R R R S R Toride 1 Pi-zt R R R R R R R R R S Ouu 316 Pi-b R R R R S R R R R R R: resistant S: susceptible
The gene has been carried in Indica varieties such as Engkatek, Milek Kuning, Tjina, and Tjahaja in Indonesia and Malaysia. In Japan, TohokuIL9, a strain homozygous for the Pi-b and having a genetic background of the sensitive variety Sasanishiki, has been bred at the Miyagi Prefectural Furukawa Agriculture Experimental Station. However, the mechanism of the resistance expression has not been clarified, nor has the Pi-b gene been isolated.